The present invention relates to an exhaust gas post treatment system for internal combustion engines, such as diesel engines and gasoline engines having direct injection, that are operated with excess air.
Limited exhaust gas components, which result during combustion processes and the permissible emissions of which are continuously being lowered, include, in addition to solid particles, nitrogen oxides. To minimize these exhaust gas components with internal combustion engines operated in motor vehicles, various methods are presently used. The reduction of the nitrogen oxides generally occurs with the aid of catalysts, and in oxygen-rich exhaust gas a reduction agent is additionally required in order to increase the selectivity and NOx conversions. These methods have become know by the collective term SCR processes, whereby SCR stands for “Selective Catalytic Reduction”. It has been used for many years in the power plant industry, and in recent times also with internal combustion engines. A detailed illustration of such processes can be found in DE 34 28 232 A1. V2O5-containing mixed oxides, for example in the form of V2O5/WO3/TiO2, can be used as SCR catalysts. Typical V2O5 proportions are between 0.2-3%. In practice ammonia, or compounds that release ammonia, such as urea or ammonium formate, in solid or dissolved form, are used as reduction agents. To convert one mol nitric oxide one mol ammonia is required.4NO+4NH3+O2→4N2+6H2O  (1)
If a platinum-containing NO-oxidation catalytic converter is disposed upstream of the SCR catalyst for the formation of NO2,2NO+O2⇄2NO2  (2)
The SCR reaction can be considerably accelerated, and the low temperature activity can be significantly raised.NO+2NH3+NO2→2N2+3H2O  (3)
With internal combustion engines operated in vehicles the nitrogen oxide reduction with the aid of the SCR process is difficult for the reason that changing operating conditions exist that make the quantitative metering of the reduction agent difficult. Although on the one hand as high a conversion of the nitrogen oxides as possible should be achieved, on the other hand care must be taken that there is no emission of unused ammonia. To provide a remedial measure, an ammonia-blocking catalytic converter is frequently disposed downstream of the SCR catalyst to convert excess ammonia into nitrogen and water vapor. The use of V2O5 as active material for the SCR catalyst can also pose a problem if the exhaust gas temperature at the SCR catalyst is greater than 650° C., because V2O5 then sublimates. For this reason, iron or copper zeolites that are free of V2O5 are used for high temperature applications.
To minimize the very fine particles, not only in the power plant industry but also with vehicles, either so-called particle separators or particle filters are used. A typical arrangement having particle separators for use in vehicles is described, for example, in EP 1 072 765 A1. Such arrangements differ from those having particle filters in that the diameter of the channels of the particle separator are considerably greater than the diameter of the largest particle that is present, whereas with particle filters the diameter of the filter channels is in the range of the diameter of the particles. As a consequence of this difference, particle filters are subject to becoming clogged, which increases the exhaust gas counter pressure and reduces the engine power. An arrangement and a method using particle filters can be found in EP 0 341 832 A2. The two aforementioned arrangements or methods are characterized in that the oxidation catalytic converter, which is respectively disposed upstream of the particle separator or particle filter, and which is generally a catalyst having platinum as the active material, oxidizes the nitric oxide in the exhaust gas with the aid of the also-contained residual oxygen to nitrogen dioxide, which inturn is converted in the particle separator, or the particle filter, with the carbon particles to CO, CO2, N2, and NO. In this way, a continuous removal of the accumulated very fine particles is effected; expensive and/or complicated regeneration cycles, which must be carried out with other arrangements, are thereby eliminated.2NO2+C→2NO+CO2  (4)2NO2+C→2NO+CO  (5)2C+2NO2→N2+2CO2  (6)
In order to fulfill the exhaust gas regulations that will be applicable in the future it is necessary to simultaneously use not only arrangements for reducing nitrogen oxide emissions, but also arrangements for reducing the emission of very fine particles. For this purpose, already various arrangements and methods have become known.
DE 103 48 799 A1 describes an arrangement that is comprised of an oxidation catalytic converter, an SCR catalyst disposed downstream thereof in the exhaust gas stream, and a particle filter that in turn is disposed downstream of the SOP catalyst in the exhaust gas stream. The supply of the reduction agent for the selective catalytic reaction that takes place in the SCR catalyst is effected immediately prior to the SCR catalyst via a urea injection device that is controlled as a function of operating parameters of the internal combustion engine. The drawback of this arrangement is that the nitrogen dioxide produced in the oxidation catalytic converter is essentially completely used up by the selective catalytic reduction in the SCR catalyst, in other words, is no longer available for the conversion of the very fine particles that have accumulated in the downstream particle filter. The regeneration of the particle filter must therefore be realized by an expensive and/or cyclical heating-up of the exhaust gas stream by enriching the exhaust gas stream with non-combusted hydrocarbons. This occurs either by enriching the combustion mixture or introducing fuel ahead of the particle filter. Such an arrangement for regenerating the particle filter is on the one hand complicated and hence expensive, and on the other hand the cyclical regeneration of the particle filter disposed at the end of the arrangement again produces harmful materials that can no longer be removed from the exhaust gas. In addition, if particle filters are used they can become clogged with oil ash, so that such filters must be removed at certain intervals and must be cleaned.
A further combination of a particle filter and an arrangement for the selective catalytic reduction is known from EP 1 054 722 A1. The arrangement described therein comprises an oxidation catalytic converter that is disposed in the exhaust gas stream and that increases the proportion of nitrogen dioxide in the exhaust gas, a fine material filter disposed downstream thereof, a reservoir for the reduction liquid, as well as an injection device for the reduction fluid that is disposed behind the fine material filter, and additionally an SCR catalytic converter disposed downstream in the exhaust gas stream. Although the above-described arrangement permits a continuous conversion of the fine material particles of the carbon type accumulated in the fine material filter with the aid of the nitrogen dioxide produced in the oxidation catalytic converter, it has another very serious drawback. The particle filter causes a cooling of the exhaust gas, so that for example with the use of the presently commercially available reduction liquid designated AdBlue, the exhaust gas temperature, in particular after startup of the internal combustion engine, or during operation of the internal combustion engine in a lower output range, is too low to produce ammonia without yielding problematic byproducts from the 33% aqueous urea solution.
In conjunction with the decomposition of urea ((NH2)2CO) into ammonia (NH3), it is known that this occurs under optimum conditions (temperatures greater than 350° C.) in two stages; according to(NH2)2CO→NH3+HNCO  (7)there is first effected the thermolysis, or pyrolysis, i.e. the thermal decomposition of urea. Subsequently, according toHNCO+H2O→NH3+CO2  (8)there is effected the hydrolysis, in other words, the catalytic conversion of isocyanic acid (HNCO) into ammonia (NH3) and carbon dioxide (CO2).
Since with the use of AdBlue the reduction agent is present in a form dissolved in water, this water must be evaporated prior to and during the actual pyrolysis and hydrolysis.
If the temperature present with the previous reaction according to (7) and (8) is less than 350° C., or is heated only slowly, it is known from DE 40 38 054 A1 that essentially solid, non-meltable cyanuric acid results from trimerization of the isocyanic acid formed according to (7) pursuant to3HNCO<350° C.→⇄>350° C.(HNCO)3  (9)which leads to clogging of the following SCR catalytic converter. Remedial action can, as outlined in the aforementioned DE 40 38 054, be provided by guiding the exhaust gas stream that is laden with the reduction agent over a hydrolysis catalytic converter. The exhaust gas temperature from which a quantitative hydrolysis is possible can thus be depressed to 160° C. The construction and composition of an appropriate catalytic converter is also described in the aforementioned publication as are the construction and function of an SCR catalytic converter system that is equipped with a hydrolysis catalytic converter.
Proceeding from the previously described state of the art, it is an object of the present invention, while avoiding the drawbacks of the known arrangements, to provide an exhaust gas post treatment system that reduces not only the particle emission but also the nitrogen oxide emission and that on the one hand operates without special regeneration cycles for the particle separator or particle filter and on the other hand, at possibly all operating conditions, releases NH3 from the reduction agent without the formation of problematic reaction products.